Mirroring splitter meta data

ABSTRACT

A method, system and computer program product for data replication comprising receiving an IO at a first storage processor (SP), sending metadata corresponding to the IO to a second SP, receiving an acknowledgement from the second SP indicating the second SP received the metadata and send the IO down an IO stack.

A portion of the disclosure of this patent document may contain commandformats and other computer language listings, all of which are subjectto copyright protection. The copyright owner has no objection to thefacsimile reproduction by anyone of the patent document or the patentdisclosure, as it appears in the Patent and Trademark Office patent fileor records, but otherwise reserves all copyright rights whatsoever.

TECHNICAL FIELD

This invention relates to data replication.

BACKGROUND

Computer data is vital to today's organizations, and a significant partof protection against disasters is focused on data protection. Assolid-state memory has advanced to the point where cost of memory hasbecome a relatively insignificant factor, organizations can afford tooperate with systems that store and process terabytes of data.

Conventional data protection systems include tape backup drives, forstoring organizational production site data on a periodic basis. Suchsystems suffer from several drawbacks. First, they require a systemshutdown during backup, since the data being backed up cannot be usedduring the backup operation. Second, they limit the points in time towhich the production site can recover. For example, if data is backed upon a daily basis, there may be several hours of lost data in the eventof a disaster. Third, the data recovery process itself takes a longtime.

Another conventional data protection system uses data replication, bycreating a copy of the organization's production site data on asecondary backup storage system, and updating the backup with changes.The backup storage system may be situated in the same physical locationas the production storage system, or in a physically remote location.Data replication systems generally operate either at the applicationlevel, at the file system level, or at the data block level.

Current data protection systems try to provide continuous dataprotection, which enable the organization to roll back to any specifiedpoint in time within a recent history. Continuous data protectionsystems aim to satisfy two conflicting objectives, as best as possible;namely, (i) minimize the down time, in which the organization productionsite data is unavailable, during a recovery, and (ii) enable recovery asclose as possible to any specified point in time within a recenthistory.

Continuous data protection typically uses a technology referred to as“journaling,” whereby a log is kept of changes made to the backupstorage. During a recovery, the journal entries serve as successive“undo” information, enabling rollback of the backup storage to previouspoints in time. Journaling was first implemented in database systems,and was later extended to broader data protection.

One challenge to continuous data protection is the ability of a backupsite to keep pace with the data transactions of a production site,without slowing down the production site. The overhead of journalinginherently requires several data transactions at the backup site foreach data transaction at the production site. As such, when datatransactions occur at a high rate at the production site, the backupsite may not be able to finish backing up one data transaction beforethe next production site data transaction occurs. If the production siteis not forced to slow down, then necessarily a backlog of un-logged datatransactions may build up at the backup site. Without being able tosatisfactorily adapt dynamically to changing data transaction rates, acontinuous data protection system chokes and eventually forces theproduction site to shut down.

SUMMARY

A method, system and computer program product for data replicationcomprising receiving an IO at a first storage processor (SP), sendingmetadata corresponding to the IO to a second SP, receiving anacknowledgement from the second SP indicating the second SP received themetadata and send the IO down an IO stack.

BRIEF DESCRIPTION OF THE DRAWINGS

Objects, features, and advantages of embodiments disclosed herein may bebetter understood by referring to the following description inconjunction with the accompanying drawings. The drawings are not meantto limit the scope of the claims included herewith. For clarity, notevery element may be labeled in every figure. The drawings are notnecessarily to scale, emphasis instead being placed upon illustratingembodiments, principles, and concepts. Thus, features and advantages ofthe present disclosure will become more apparent from the followingdetailed description of exemplary embodiments thereof taken inconjunction with the accompanying drawings in which:

FIG. 1 is a simplified illustration of a data protection system, inaccordance with an embodiment of the present disclosure;

FIG. 2 is a simplified illustration of a write transaction for ajournal, in accordance with an embodiment of the present disclosure;

FIG. 3 is a simplified illustration of a storage system with two storageprocessors and storage mediums, in accordance with an embodiment of thepresent disclosure;

FIG. 4 is a simplified illustration of a storage system with two storageprocessors and a LUN and two replication protection appliances, inaccordance with an embodiment of the present disclosure;

FIG. 5 is a simplified method for acknowledging replicated IO, inaccordance with an embodiment of the present disclosure;

FIG. 6 is a simplified method for flushing IO meta data, in accordancewith an embodiment of the present disclosure;

FIG. 7 is a simplified method for resyncing data after a splitterfailure, in accordance with an embodiment of the present disclosure;

FIG. 8 is a simplified method for re-syncing data, in accordance with anembodiment of the present disclosure;

FIG. 9 is an example of an embodiment of an apparatus that may utilizethe techniques described herein, in accordance with an embodiment of thepresent disclosure; and

FIG. 10 is an example of an embodiment of a method embodied on acomputer readable storage medium that may utilize the techniquesdescribed herein, in accordance with an embodiment of the presentdisclosure.

DETAILED DESCRIPTION

Thus, a typical storage system may provide high availability to the datawhich is stored. Generally, a storage array may have at least twostorage processors (SP) to enable one of the SPs to fail while stillenabling the data storage system to function. Conventionally, the SPsare connected through a link with high bandwidth and low latency.Generally, a failure of one element in a storage system such as storageprocessor, or data protection appliance should not have a significantimpact on the data path, i.e. the ability to write or read data.

Typically however, if data marking the execution of the read and writeIO is lost, the system may need to do a full sweep of the data in orderto make sure that the production and replica storage device areidentical. Usually, a full sweep is an expensive procedure which readsall the data from the production storage array and compares it to thereplica storage array data. Conventionally, a replication product wherea failure of one component such as data protection appliance (DPA) orstorage processor causes a complete full sweep of the data causesexcessive down time, which negatively impacts the data center and isundesirable for users of the data storage system.

In conventional implementations, data is first sent to the DPA and thensent down the storage IO stack. In this conventional implementation, ifthe storage processor fails the data may have already arrived to theDPA. Typically, the DPA knows which IO arrived to the storage regardlessif the IO completed or not. Usually, after a SP crashes, if noacknowledgement has reached the host, there may be no way to know if theIO was written to the backend devices, the DPA may re-sync the data ofthe latest IO operations, as it may not be known if the IOs completed.Generally, if the DPA crashes the splitter may know which IOs where inthe cache of the DPA due to a protocol between the splitter and the DPA.Conventionally, the disadvantages of this method may be that the IOlatency is additive; i.e. a better approach may be to send the IO toboth the DPA and the backend storage at the same time. Typically, inorder to be able to split the data at the same time there may need to bemechanism which lets the system track the incoming IOs even if onecomponent of the storage system or replication appliance is lost.

In an embodiment, the current disclosure may enable leveraging of thetwo SP of a storage array during replication to minor each IO meta datain both SPs. In some embodiments, a splitter in one of the SPs mayintercept IO. In certain embodiments, the splitter may send metadatacorresponding to the IO to the second SP through the interconnectbetween the SPs. In some embodiments, after metadata acknowledged at tothe first SP by the second SP, the IO may be sent to both thereplication appliance, and the IO stack. In most embodiments, the IO maybe sent in any order to the replication appliance and down the IO stackincluding being sent to both at the same time. In further embodiments,in case of a failure of the splitter, the metadata of the second SP mayinclude locations which may have not been written to the replicationappliance. In at least some embodiments, the replication appliance mayread the dirty locations from the second SP to enable resynchronizationof data which may have not been replicated correctly due to the failure.In case of a replication appliance failure, the secondary replicationappliance may take the role of the failed replication appliance, mayread the tracked meta data from both SPs, in order to resynchronize datawhich might have been in the cache of the failed replication appliance.

The following definitions are employed throughout the specification andclaims.

BACKUP SITE—may be a facility where replicated production site data isstored; the backup site may be located in a remote site or at the samelocation as the production site;

CLONE—a clone may be a copy or clone of the image or images, drive ordrives of a first location at a second location;

DELTA MARKING STREAM—may mean the tracking of the delta between theproduction and replication site, which may contain the meta data ofchanged locations, the delta marking stream may be kept persistently onthe journal at the production site of the replication, based on thedelta marking data the DPA knows which locations are different betweenthe production and the replica and transfers them to the replica to makeboth sites identical.

DPA—may be Data Protection Appliance a computer or a cluster ofcomputers, or a set of processes that serve as a data protectionappliance, responsible for data protection services including inter aliadata replication of a storage system, and journaling of I/O requestsissued by a host computer to the storage system;

RPA—may be replication protection appliance, is another name for DPA.

HOST—may be at least one computer or networks of computers that runs atleast one data processing application that issues I/O requests to one ormore storage systems; a host is an initiator with a SAN;

HOST DEVICE—may be an internal interface in a host, to a logical storageunit;

IMAGE—may be a copy of a logical storage unit at a specific point intime;

INITIATOR—may be a node in a SAN that issues I/O requests;

JOURNAL—may be a record of write transactions issued to a storagesystem; used to maintain a duplicate storage system, and to rollback theduplicate storage system to a previous point in time;

LOGICAL UNIT—may be a logical entity provided by a storage system foraccessing data from the storage system;

LUN—may be a logical unit number for identifying a logical unit;PHYSICAL STORAGE UNIT—may be a physical entity, such as a disk or anarray of disks, for storing data in storage locations that can beaccessed by address;

PRODUCTION SITE—may be a facility where one or more host computers rundata processing applications that write data to a storage system andread data from the storage system;

SAN—may be a storage area network of nodes that send and receive I/O andother requests, each node in the network being an initiator or a target,or both an initiator and a target;

SOURCE SIDE—may be a transmitter of data within a data replicationworkflow, during normal operation a production site is the source side;and during data recovery a backup site is the source side;

SNAPSHOT—a Snapshot may refer to differential representations of animage, i.e. the snapshot may have pointers to the original volume, andmay point to log volumes for changed locations. Snapshots may becombined into a snapshot array, which may represent different imagesover a time period.

STORAGE SYSTEM—may be a SAN entity that provides multiple logical unitsfor access by multiple SAN initiators

TARGET—may be a node in a SAN that replies to I/O requests;

TARGET SIDE—may be a receiver of data within a data replicationworkflow; during normal operation a back site is the target side, andduring data recovery a production site is the target side;

WAN—may be a wide area network that connects local networks and enablesthem to communicate with one another, such as the Internet.

SPLITTER/PROTECTION AGENT: may be an agent running either on aproduction host a switch or a storage array which can intercept IO andsplit them to a DPA and to the storage array, fail IO redirect IO or doany other manipulation to the IO.

VIRTUAL VOLUME: may be a volume which is exposed to host by avirtualization layer, the virtual volume may be spanned across more thanone site

DISTRIBUTED MIRROR: may be a mirror of a volume across distance, eithermetro or geo, which is accessible at all sites.

BLOCK VIRTUALIZATION: may be a layer, which takes backend storagevolumes and by slicing concatenation and striping create a new set ofvolumes, which serve as base volumes or devices in the virtualizationlayer

MARKING ON SPLITTER: may be a mode in a splitter where intercepted IOsare not split to an appliance and the storage, but changes (meta data)are tracked in a list and/or a bitmap and I/O is immediately sent todown the IO stack.

FAIL ALL MODE: may be a mode of a volume in the splitter where all writeand read IOs intercepted by the splitter are failed to the host, butother SCSI commands like read capacity are served.

GLOBAL FAIL ALL MODE: may be a mode of a volume in the virtual layerwhere all write and read IOs virtual layer are failed to the host, butother SCSI commands like read capacity are served.

LOGGED ACCESS: may be an access method provided by the appliance and thesplitter, in which the appliance rolls the volumes of the consistencygroup to the point in time the user requested and let the host accessthe volumes in a copy on first write base.

VIRTUAL ACCESS: may be an access method provided by the appliance andthe splitter, in which the appliance exposes a virtual volume from aspecific point in time to the host, the data for the virtual volume ispartially stored on the remote copy and partially stored on the journal.

CDP: Continuous Data Protection, may refer to a full replica of a volumeor a set of volumes along with a journal which allows any point in timeaccess, the CDP copy is at the same site, and maybe the same storagearray of the production site

CRR: Continuous Remote Replica may refer to a full replica of a volumeor a set of volumes along with a journal which allows any point in timeaccess at a site remote to the production volume and on a separatestorage array.

As used herein, the term storage medium may refer to one or more storagemediums such as a hard drive, a combination of hard drives, flashstorage, combinations of flash storage, combinations of hard drives,flash, and other storage devices, and other types and combinations ofcomputer readable storage mediums including those yet to be conceived. Astorage medium may also refer both physical and logical storage mediumsand may include multiple level of virtual to physical mappings and maybe or include an image or disk image.

A description of journaling and some techniques associated withjournaling may be described in the patent titled “METHODS AND APPARATUSFOR OPTIMAL JOURNALING FOR CONTINUOUS DATA REPLICATION” and with U.S.Pat. No. 7,516,287, which is hereby incorporated by reference.

A discussion of image access may be found in U.S. patent applicationSer. No. 12/969,903 entitled “DYNAMIC LUN RESIZING IN A REPLICATIONENVIRONMENT” filed on Apr. 16, 2010 assigned to EMC Corp., which ishereby incorporated by reference.

A discussion of storage systems and storage system redundancy may befound in U.S. Pat. No. 7,209,979 entitled “STORAGE PROCESSORARCHITECTURE FOR HIGH THROUGHPUT APPLICATIONS PROVIDING EFFICIENT USERDATA CHANNEL LOADING” issued on Apr. 24, 2007 assigned to EMC Corp.,which is hereby incorporated by reference.

A discussion of storage systems and storage system redundancy may alsobe found in U.S. patent application Ser. No. 13/172,517 entitled“SELECTING PHYSICAL STORAGE IN DATA STORAGE SYSTEMS” filed on Jun. 29,2011 and U.S. patent application Ser. No. 12/980,912 entitled “MANANGINGOWNERSHIP OF LOGICAL VOLUMES” filed on Dec. 29, 2010 both of which areassigned to EMC Corp. and are hereby incorporated by reference.

Description of Embodiments Using of a Five State Journaling Process

Reference is now made to FIG. 1, which is a simplified illustration of adata protection system 100, in accordance with an embodiment of thepresent invention. Shown in FIG. 1 are two sites; Site I, which is aproduction site, on the right, and Site II, which is a backup site, onthe left. Under normal operation the production site is the source sideof system 100, and the backup site is the target side of the system. Thebackup site is responsible for replicating production site data.Additionally, the backup site enables rollback of Site I data to anearlier pointing time, which may be used in the event of data corruptionof a disaster, or alternatively in order to view or to access data froman earlier point in time.

During normal operations, the direction of replicate data flow goes fromsource side to target side. It is possible, however, for a user toreverse the direction of replicate data flow, in which case Site Istarts to behave as a target backup site, and Site II starts to behaveas a source production site. Such change of replication direction isreferred to as a “failover”. A failover may be performed in the event ofa disaster at the production site, or for other reasons. In some dataarchitectures, Site I or Site II behaves as a production site for aportion of stored data, and behaves simultaneously as a backup site foranother portion of stored data. In some data architectures, a portion ofstored data is replicated to a backup site, and another portion is not.

The production site and the backup site may be remote from one another,or they may both be situated at a common site, local to one another.Local data protection has the advantage of minimizing data lag betweentarget and source, and remote data protection has the advantage is beingrobust in the event that a disaster occurs at the source side.

The source and target sides communicate via a wide area network (WAN)128, although other types of networks are also adaptable for use withthe present invention.

In accordance with an embodiment of the present invention, each side ofsystem 100 includes three major components coupled via a storage areanetwork (SAN); namely, (i) a storage system, (ii) a host computer, and(iii) a data protection appliance (DPA). Specifically with reference toFIG. 1, the source side SAN includes a source host computer 104, asource storage system 108, and a source DPA 112. Similarly, the targetside SAN includes a target host computer 116, a target storage system120, and a target DPA 124.

Generally, a SAN includes one or more devices, referred to as “nodes”. Anode in a SAN may be an “initiator” or a “target”, or both. An initiatornode is a device that is able to initiate requests to one or more otherdevices; and a target node is a device that is able to reply torequests, such as SCSI commands, sent by an initiator node. A SAN mayalso include network switches, such as fiber channel switches. Thecommunication links between each host computer and its correspondingstorage system may be any appropriate medium suitable for data transfer,such as fiber communication channel links.

In an embodiment of the present invention, the host communicates withits corresponding storage system using small computer system interface(SCSI) commands.

System 100 includes source storage system 108 and target storage system120. Each storage system includes physical storage units for storingdata, such as disks or arrays of disks. Typically, storage systems 108and 120 are target nodes. In order to enable initiators to send requeststo storage system 108, storage system 108 exposes one or more logicalunits (LU) to which commands are issued. Thus, storage systems 108 and120 are SAN entities that provide multiple logical units for access bymultiple SAN initiators.

Logical units are a logical entity provided by a storage system, foraccessing data stored in the storage system. A logical unit isidentified by a unique logical unit number (LUN). In an embodiment ofthe present invention, storage system 108 exposes a logical unit 136,designated as LU A, and storage system 120 exposes a logical unit 156,designated as LU B.

In an embodiment of the present invention, LU B is used for replicatingLU A. As such, LU B is generated as a copy of LU A. In one embodiment,LU B is configured so that its size is identical to the size of LU A.Thus for LU A, storage system 120 serves as a backup for source sidestorage system 108. Alternatively, as mentioned hereinabove, somelogical units of storage system 120 may be used to back up logical unitsof storage system 108, and other logical units of storage system 120 maybe used for other purposes. Moreover, in certain embodiments of thepresent invention, there is symmetric replication whereby some logicalunits of storage system 108 are used for replicating logical units ofstorage system 120, and other logical units of storage system 120 areused for replicating other logical units of storage system 108.

System 100 includes a source side host computer 104 and a target sidehost computer 116. A host computer may be one computer, or a pluralityof computers, or a network of distributed computers, each computer mayinclude inter alia a conventional CPU, volatile and non-volatile memory,a data bus, an I/O interface, a display interface and a networkinterface. Generally a host computer runs at least one data processingapplication, such as a database application and an e-mail server.

Generally, an operating system of a host computer creates a host devicefor each logical unit exposed by a storage system in the host computerSAN. A host device is a logical entity in a host computer, through whicha host computer may access a logical unit. In an embodiment of thepresent invention, host device 104 identifies LU A and generates acorresponding host device 140, designated as Device A, through which itcan access LU A. Similarly, host computer 116 identifies LU B andgenerates a corresponding device 160, designated as Device B.

In an embodiment of the present invention, in the course of continuousoperation, host computer 104 is a SAN initiator that issues I/O requests(write/read operations) through host device 140 to LU A using, forexample, SCSI commands. Such requests are generally transmitted to LU Awith an address that includes a specific device identifier, an offsetwithin the device, and a data size. Offsets are generally aligned to 512byte blocks. The average size of a write operation issued by hostcomputer 104 may be, for example, 10 kilobytes (KB); i.e., 20 blocks.For an I/O rate of 50 megabytes (MB) per second, this corresponds toapproximately 5,000 write transactions per second.

System 100 includes two data protection appliances, a source side DPA112 and a target side DPA 124. A DPA performs various data protectionservices, such as data replication of a storage system, and journalingof I/O requests issued by a host computer to source side storage systemdata. As explained in detail hereinbelow, when acting as a target sideDPA, a DPA may also enable rollback of data to an earlier point in time,and processing of rolled back data at the target site. Each DPA 112 and124 is a computer that includes inter alia one or more conventional CPUsand internal memory.

For additional safety precaution, each DPA is a cluster of suchcomputers. Use of a cluster ensures that if a DPA computer is down, thenthe DPA functionality switches over to another computer. The DPAcomputers within a DPA cluster communicate with one another using atleast one communication link suitable for data transfer via fiberchannel or IP based protocols, or such other transfer protocol. Onecomputer from the DPA cluster serves as the DPA leader. The DPA clusterleader coordinates between the computers in the cluster, and may alsoperform other tasks that require coordination between the computers,such as load balancing.

In the architecture illustrated in FIG. 1, DPA 112 and DPA 124 arestandalone devices integrated within a SAN. Alternatively, each of DPA112 and DPA 124 may be integrated into storage system 108 and storagesystem 120, respectively, or integrated into host computer 104 and hostcomputer 116, respectively. Both DPAs communicate with their respectivehost computers through communication lines such as fiber channels using,for example, SCSI commands.

In accordance with an embodiment of the present invention, DPAs 112 and124 are configured to act as initiators in the SAN; i.e., they can issueI/O requests using, for example, SCSI commands, to access logical unitson their respective storage systems. DPA 112 and DPA 124 are alsoconfigured with the necessary functionality to act as targets; i.e., toreply to I/O requests, such as SCSI commands, issued by other initiatorsin the SAN, including inter alia their respective host computers 104 and116. Being target nodes, DPA 112 and DPA 124 may dynamically expose orremove one or more logical units.

As described hereinabove, Site I and Site II may each behavesimultaneously as a production site and a backup site for differentlogical units. As such, DPA 112 and DPA 124 may each behave as a sourceDPA for some logical units, and as a target DPA for other logical units,at the same time.

In accordance with an embodiment of the present invention, host computer104 and host computer 116 include protection agents 144 and 164,respectively. Protection agents 144 and 164 intercept SCSI commandsissued by their respective host computers, via host devices to logicalunits that are accessible to the host computers. In accordance with anembodiment of the present invention, a data protection agent may act onan intercepted SCSI commands issued to a logical unit, in one of thefollowing ways:

-   -   Send the SCSI commands to its intended logical unit.    -   Redirect the SCSI command to another logical unit.    -   Split the SCSI command by sending it first to the respective        DPA. After the DPA returns an acknowledgement, send the SCSI        command to its intended logical unit.    -   Fail a SCSI command by returning an error return code.    -   Delay a SCSI command by not returning an acknowledgement to the        respective host computer.

A protection agent may handle different SCSI commands, differently,according to the type of the command. For example, a SCSI commandinquiring about the size of a certain logical unit may be sent directlyto that logical unit, while a SCSI write command may be split and sentfirst to a DPA associated with the agent. A protection agent may alsochange its behavior for handling SCSI commands, for example as a resultof an instruction received from the DPA.

Specifically, the behavior of a protection agent for a certain hostdevice generally corresponds to the behavior of its associated DPA withrespect to the logical unit of the host device. When a DPA behaves as asource site DPA for a certain logical unit, then during normal course ofoperation, the associated protection agent splits I/O requests issued bya host computer to the host device corresponding to that logical unit.Similarly, when a DPA behaves as a target device for a certain logicalunit, then during normal course of operation, the associated protectionagent fails I/O requests issued by host computer to the host devicecorresponding to that logical unit.

Communication between protection agents and their respective DPAs mayuse any protocol suitable for data transfer within a SAN, such as fiberchannel, or SCSI over fiber channel. The communication may be direct, orvia a logical unit exposed by the DPA. In an embodiment of the presentinvention, protection agents communicate with their respective DPAs bysending SCSI commands over fiber channel.

In an embodiment of the present invention, protection agents 144 and 164are drivers located in their respective host computers 104 and 116.Alternatively, a protection agent may also be located in a fiber channelswitch, or in any other device situated in a data path between a hostcomputer and a storage system.

What follows is a detailed description of system behavior under normalproduction mode, and under recovery mode.

In accordance with an embodiment of the present invention, in productionmode DPA 112 acts as a source site DPA for LU A. Thus, protection agent144 is configured to act as a source side protection agent; i.e., as asplitter for host device A. Specifically, protection agent 144replicates SCSI I/O requests. A replicated SCSI I/O request is sent toDPA 112. After receiving an acknowledgement from DPA 124, protectionagent 144 then sends the SCSI I/O request to LU A. Only after receivinga second acknowledgement from storage system 108 may host computer 104initiate another I/O request.

When DPA 112 receives a replicated SCSI write request from dataprotection agent 144, DPA 112 transmits certain I/O informationcharacterizing the write request, packaged as a “write transaction”,over WAN 128 to DPA 124 on the target side, for journaling and forincorporation within target storage system 120.

DPA 112 may send its write transactions to DPA 124 using a variety ofmodes of transmission, including inter alia (i) a synchronous mode, (ii)an asynchronous mode, and (iii) a snapshot mode. In synchronous mode,DPA 112 sends each write transaction to DPA 124, receives back anacknowledgement from DPA 124, and in turns sends an acknowledgement backto protection agent 144. Protection agent 144 waits until receipt ofsuch acknowledgement before sending the SCSI write request to LU A.

In asynchronous mode, DPA 112 sends an acknowledgement to protectionagent 144 upon receipt of each I/O request, before receiving anacknowledgement back from DPA 124.

In snapshot mode, DPA 112 receives several I/O requests and combinesthem into an aggregate “snapshot” of all write activity performed in themultiple I/O requests, and sends the snapshot to DPA 124, for journalingand for incorporation in target storage system 120. In snapshot mode DPA112 also sends an acknowledgement to protection agent 144 upon receiptof each I/O request, before receiving an acknowledgement back from DPA124.

For the sake of clarity, the ensuing discussion assumes that informationis transmitted at write-by-write granularity.

While in production mode, DPA 124 receives replicated data of LU A fromDPA 112, and performs journaling and writing to storage system 120. Whenapplying write operations to storage system 120, DPA 124 acts as aninitiator, and sends SCSI commands to LU B.

During a recovery mode, DPA 124 undoes the write transactions in thejournal, so as to restore storage system 120 to the state it was at, atan earlier time.

As described hereinabove, in accordance with an embodiment of thepresent invention, LU B is used as a backup of LU A. As such, duringnormal production mode, while data written to LU A by host computer 104is replicated from LU A to LU B, host computer 116 should not be sendingI/O requests to LU B. To prevent such I/O requests from being sent,protection agent 164 acts as a target site protection agent for hostDevice B and fails I/O requests sent from host computer 116 to LU Bthrough host Device B.

In accordance with an embodiment of the present invention, targetstorage system 120 exposes a logical unit 176, referred to as a “journalLU”, for maintaining a history of write transactions made to LU B,referred to as a “journal”. Alternatively, journal LU 176 may be stripedover several logical units, or may reside within all of or a portion ofanother logical unit. DPA 124 includes a journal processor 180 formanaging the journal.

Journal processor 180 functions generally to manage the journal entriesof LU B. Specifically, journal processor 180 (i) enters writetransactions received by DPA 124 from DPA 112 into the journal, bywriting them into the journal LU, (ii) applies the journal transactionsto LU B, and (iii) updates the journal entries in the journal LU withundo information and removes already-applied transactions from thejournal. As described below, with reference to FIGS. 2 and 3A-3D,journal entries include four streams, two of which are written whenwrite transaction are entered into the journal, and two of which arewritten when write transaction are applied and removed from the journal.

Reference is now made to FIG. 2, which is a simplified illustration of awrite transaction 200 for a journal, in accordance with an embodiment ofthe present invention. The journal may be used to provide an adaptor foraccess to storage 120 at the state it was in at any specified point intime. Since the journal contains the “undo” information necessary torollback storage system 120, data that was stored in specific memorylocations at the specified point in time may be obtained by undoingwrite transactions that occurred subsequent to such point in time.

Write transaction 200 generally includes the following fields:

-   -   one or more identifiers;    -   a time stamp, which is the date & time at which the transaction        was received by source side DPA 112;    -   a write size, which is the size of the data block;    -   a location in journal LU 176 where the data is entered;    -   a location in LU B where the data is to be written; and    -   the data itself.

Write transaction 200 is transmitted from source side DPA 112 to targetside DPA 124. As shown in FIG. 2, DPA 124 records the write transaction200 in four streams. A first stream, referred to as a DO stream,includes new data for writing in LU B. A second stream, referred to asan DO METADATA stream, includes metadata for the write transaction, suchas an identifier, a date & time, a write size, a beginning address in LUB for writing the new data in, and a pointer to the offset in the dostream where the corresponding data is located. Similarly, a thirdstream, referred to as an UNDO stream, includes old data that wasoverwritten in LU B; and a fourth stream, referred to as an UNDOMETADATA, include an identifier, a date & time, a write size, abeginning address in LU B where data was to be overwritten, and apointer to the offset in the undo stream where the corresponding olddata is located.

In practice each of the four streams holds a plurality of writetransaction data. As write transactions are received dynamically bytarget DPA 124, they are recorded at the end of the DO stream and theend of the DO METADATA stream, prior to committing the transaction.During transaction application, when the various write transactions areapplied to LU B, prior to writing the new DO data into addresses withinthe storage system, the older data currently located in such addressesis recorded into the UNDO stream.

By recording old data, a journal entry can be used to “undo” a writetransaction. To undo a transaction, old data is read from the UNDOstream in a reverse order, from the most recent data to the oldest data,for writing into addresses within LU B. Prior to writing the UNDO datainto these addresses, the newer data residing in such addresses isrecorded in the DO stream.

The journal LU is partitioned into segments with a pre-defined size,such as 1 MB segments, with each segment identified by a counter. Thecollection of such segments forms a segment pool for the four journalingstreams described hereinabove. Each such stream is structured as anordered list of segments, into which the stream data is written, andincludes two pointers—a beginning pointer that points to the firstsegment in the list and an end pointer that points to the last segmentin the list.

According to a write direction for each stream, write transaction datais appended to the stream either at the end, for a forward direction, orat the beginning, for a backward direction. As each write transaction isreceived by DPA 124, its size is checked to determine if it can fitwithin available segments. If not, then one or more segments are chosenfrom the segment pool and appended to the stream's ordered list ofsegments.

Thereafter the DO data is written into the DO stream, and the pointer tothe appropriate first or last segment is updated. Freeing of segments inthe ordered list is performed by simply changing the beginning or theend pointer. Freed segments are returned to the segment pool for re-use.

A journal may be made of any number of streams including less than ormore than 5 streams. Often, based on the speed of the journaling andwhether the back-up is synchronous or a synchronous a fewer or greaternumber of streams may be used.

Delta Marking

A delta marker stream may contain the locations that may be differentbetween the latest I/O data which arrived to the remote side (thecurrent remote site) and the latest I/O data which arrived at the localside. In particular, the delta marking stream may include metadata ofthe differences between the source side and the target side. Forexample, every I/O reaching the data protection appliance for the source112 may be written to the delta marking stream and data is freed fromthe delta marking stream when the data safely arrives at both the sourcevolume of replication 108 and the remote journal 180 (e.g. DO stream).Specifically, during an initialization process no data may be freed fromthe delta marking stream; and only when the initialization process iscompleted and I/O data has arrived to both local storage and the remotejournal data, may be I/O data from the delta marking stream freed. Whenthe source and target are not synchronized, data may not be freed fromthe delta marking stream. The initialization process may start bymerging delta marking streams of the target and the source so that thedelta marking stream includes a list of all different locations betweenlocal and remote sites. For example, a delta marking stream at thetarget might have data too if a user has accessed an image at the targetsite.

The initialization process may create one virtual disk out of all theavailable user volumes. The virtual space may be divided into a selectednumber of portions depending upon the amount of data needed to besynchronized. A list of ‘dirty’ blocks may be read from the delta markerstream that is relevant to the area currently being synchronized toenable creation of a dirty location data structure. The system may beginsynchronizing units of data, where a unit of data is a constant amountof dirty data, e.g., a data that needs to be synchronized.

The dirty location data structure may provide a list of dirty locationuntil the amount of dirty location is equal to the unit size or untilthere is no data left. The system may begin a so-called ping pongprocess to synchronize the data. The process may transfer thedifferences between the production and replica site to the replica.

Data Storage System

Refer now to the example embodiment of FIG. 3. In the example embodimentof FIG. 3, host 310 may communicate IO 340 to storage system 305.Storage system 305 has two storage processors, SPA 312 and SPB 316. Thestorage processors may communicate with a storage medium such as diskdrives 325. IO 340 communicated from host 310 may be sent throughstorage processor SPA 312 or SPB 316 to Disk Drives 325. Storageprocessors SPA 312 and SPB 316 are connected by link 335. In someembodiments, the link may be quick link, such as a fiber connection, orshared PCI bus. In certain embodiments SPA 312 and SPB 316 may mirror IObetween the storage processors to provide IO redundancy.

Refer now to the example embodiment of FIG. 4. In the example embodimentof FIG. 4, each storage processor, Storage processor SPA 412 and storageprocessor SPB 416 have a splitter, splitter 415 and splitter 420respectively. Each storage processor SPA 412 and SPB 416 may expose LUN425. In this embodiment, SPA 412 and SPB 414 expose LUN 425 as a logicalunit created on the physical spindles or disks of FIG. 3. Storage system405 may also communicate with RPA 430.

Refer now to the example embodiments of FIGS. 4 and 5. In these exampleembodiments, Splitter 415 intercepts IO 440 sent from host 410 tostorage system 405 (step 505). Splitter 415 of SPA 412 sends metadata442 corresponding to IO 440 to splitter 420 (step 520). Splitter 420receives metadata 442 from splitter 415 (step 540) and acknowledge themeta data. Splitter 415 sends IO 440 down the IO stack to LUN 425 (step550). In certain embodiments, the IO may be sent to the RPA and down theIO stack co-temporaneously i.e. at the same time. Splitter 415 sends IO440 to the RPA (step 560). Given an acknowledgement of IO 440 sent byRPA 430 and acknowledgement from the IO stack, an acknowledgement may besent to the application on host 425. Splitter 420 may be maintaining alist of the metadata sent to it by splitter 415.

Refer now to the example embodiment of FIGS. 4 and 6. Periodically,Splitter 415 sends a flush start command to splitter 420 (step 605).Splitter 420 puts a marker in the list of meta data (step 620). Splitter415 confirms that RPA 430 has written the IOs meta data (step 640) tothe delta marker stream, the confirmation is done through a controlcommunication channel which may also be implemented through fiberchannel protocol or through any other protocol enabling the devices tocommunicate. Splitter 415 sends a flush end command to splitter 420(step 650). Splitter 420 may free the metadata list up to the latestpoint marked by the flush meta data start command (step 660). Theembodiments of FIGS. 4 and 6 may ensure, if SPA crashes, that the metadata of the latest IOs which may not have arrived to the replica site iseither in the delta marker stream maintained by DPA 430 or in the cacheof splitter 420.

Refer now to the example embodiment of FIGS. 4 and 7, which describe anembodiment of the recovery from a failure of splitter 420 in SPA 412. Adetermination is made if splitter is down (step 705). Replication isstopped (step 720). The list of metadata is read (step 740) and the metadata read is added to the delta marking stream of the consistency groupby DPA 430. Data is synched to the appliance based on the metadata listand the data in the delta marking stream (step 750). In mostembodiments, the storage processors function symmetrically. Thus, if SPB416 were to fail, it may be handled in the same way as a failure of SPA412.

Refer now to the example embodiments of FIGS. 4 and 8. In these exampleembodiments, DPA 430 has failed. DPA 431 may begin replicating volume425 for failed DPA 430. Meta data is read from both splitter 415 in SPA412 and splitter 420 in SPB 416 (step 805). The metadata is added to thedelta marking stream, written in journal maintained by the DPAreplicating volume 425 (step 820). DPA 431 re-syncs the data (step 840).

The methods and apparatus of this invention may take the form, at leastpartially, of program code (i.e., instructions) embodied in tangiblenon-transitory media, such as floppy diskettes, CD-ROMs, hard drives,random access or read only-memory, or any other machine-readable storagemedium. When the program code is loaded into and executed by a machine,such as the computer of FIG. 9, the machine becomes an apparatus forpracticing the invention. When implemented on one or moregeneral-purpose processors, the program code combines with such aprocessor to provide a unique apparatus that operates analogously tospecific logic circuits. As such a general purpose digital machine canbe transformed into a special purpose digital machine. FIG. 10 showsProgram Logic 1010 embodied on a computer-readable medium 1030 as shown,and wherein the Logic is encoded in computer-executable code configuredfor carrying out the reservation service process of this invention andthereby forming a Computer Program Product 1000. Logic 940 of FIG. 9 maybe loaded into memory 904 and executed by processor 930. Logic 940 mayalso be the same logic 1010 on computer readable medium 1030.

The logic for carrying out the method may be embodied as part of theaforementioned system, which is useful for carrying out a methoddescribed with reference to embodiments shown in, for example, FIG. 1and FIG. 2. For purposes of illustrating the present invention, theinvention is described as embodied in a specific configuration and usingspecial logical arrangements, but one skilled in the art may appreciatethat the device is not limited to the specific configuration but ratheronly by the claims included with this specification.

Although the foregoing invention has been described in some detail forpurposes of clarity of understanding, it may be apparent that certainchanges and modifications may be practiced within the scope of theappended claims. Accordingly, the present implementations are to beconsidered as illustrative and not restrictive, and the invention is notto be limited to the details given herein, but may be modified withinthe scope and equivalents of the appended claims.

In reading the above description, persons skilled in the art willrealize that there are many apparent variations that can be applied tothe methods and systems described. Thus it will be appreciated that, inaddition to data replication systems, the optimal journaling policy ofthe present invention has widespread application to journaling systemsincluding database systems and version control systems.

In the foregoing specification, the invention has been described withreference to specific exemplary embodiments thereof. It may, however, beevident that various modifications and changes may be made to thespecific exemplary embodiments without departing from the broader spiritand scope of the invention as set forth in the appended claims.Accordingly, the specification and drawings are to be regarded in anillustrative rather than a restrictive sense.

What is claimed is:
 1. A system for data replication, the systemcomprising: a storage system, the storage system comprising a storagemedium, a first storage processor (SP) having a first splitter, a secondSP having a second splitter; and computer-executable logic operating inmemory, wherein the computer-executable program logic is configured forexecution of: receiving an IO at the first SP; sending metadatacorresponding to the IO to the second SP; receiving an acknowledgementfrom the second SP indicating the second SP received the metadata; andsend the IO down an IO stack of the storage system.
 2. The system ofclaim 1 further comprising a data protection appliance (DPA) and whereinthe computer executable logic is further configured for execution of:sending the IO to the DPA.
 3. The system of claim 2 wherein the computerexecutable logic is further configured for execution of: sending, fromthe first SP, a flush marker to the second SP; putting the flush markerinto the metadata list at the second SP; confirming the DPA write themeta data up to the flush marker; and send the a flush metadata endcommand to the second SP.
 4. The system of claim 2 wherein the computerexecutable logic is further configured for execution of: determiningthat a splitter is down; stopping replication; reading metadata in thesplitter that has not failed; and re-syncing data to the DPA based onthe metadata.
 5. The system of claim 2 further comprising a second DPAand wherein the computer executable logic is further configured forexecution of: determining that a DPA is down; stopping replication;reading metadata in the splitters; and re-syncing data to the second DPAbased on the metadata.
 6. The system of claim 3 further comprising asecond DPA and wherein the computer executable logic is furtherconfigured for execution of: freeing, by the splitter at the secondstorage processor, the metadata before the flush marker.
 7. A computerprogram product for use in replication comprising: a non-transitorycomputer readable medium encoded with computer executable program codefor data replication, the code configured to enable the execution of:receiving an IO at a first storage processor (SP) of a storage system,wherein the storage system has a second SP; sending metadatacorresponding to the IO to the second SP; receiving an acknowledgementfrom the second SP indicating the second SP received the metadata; andsend the IO down an IO stack of the storage system.
 8. The programproduct of claim 7 wherein the executable program code is furtherconfigured for execution of sending the IO to a DPA.
 9. The programproduct of claim 8 wherein the executable program code is furtherconfigured for execution of: sending, from the first SP, a flush markerto the second SP; putting the flush marker into the metadata list at thesecond SP; confirming the DPA write the meta data up to the flushmarker; and send the a flush metadata end command to the second SP. 10.The program product of claim 8 wherein the executable program code isfurther configured for execution of: determining that a splitter isdown; stopping replication; reading metadata in the splitter that hasnot failed; and re-syncing data to the DPA based on the metadata. 11.The program product of claim 8 wherein the executable program code isfurther configured for execution of: determining that a DPA is down;stopping replication; reading metadata in the splitters; and re-syncingdata to the second DPA based on the metadata.
 12. The program product ofclaim 9 wherein the executable program code is further configured forexecution of: freeing, by the splitter at the second storage processor,the metadata before the flush marker.
 13. A computer implemented methodfor data replication, the method comprising: receiving an IO at a firststorage processor (SP) of a storage system, wherein the storage systemhas a second SP; sending metadata corresponding to the IO to the secondSP; receiving an acknowledgement from the second SP indicating thesecond SP received the metadata; and send the IO down an IO stack of thestorage system.
 14. The computer implemented method of claim 13 whereinthe method further comprising: sending the IO to a DPA.
 15. The computerimplemented method of claim 14 wherein the method further comprising:sending, from the first SP, a flush marker to the second SP; putting theflush marker into the metadata list at the second SP; confirming the DPAwrite the meta data up to the flush marker; and send the a flushmetadata end command to the second SP.
 16. The computer implementedmethod of claim 14 wherein the method further comprising: determiningthat a splitter is down; stopping replication; reading metadata in thesplitter that has not failed; and re-syncing data to the DPA based onthe metadata.
 17. The computer implemented method of claim 14 whereinthe method further comprising: determining that a DPA is down; stoppingreplication; reading metadata in the splitters; and re-syncing data tothe second DPA based on the metadata.
 18. The computer implementedmethod of claim 14 wherein the method further comprising: freeing by thesplitter at the second storage processor, the metadata before the flushmarker.